One who posts content on a forum to make people extremely angry, and possibly start a flame war. In recent years however, trolling has greatly evolved, spawning entire categories of ways to piss people off. Trolls are also largely shaped by their internet environment, such that on forums, they may merely "flame", but on other sites such as anonymous chat sites (such as omegle) they go to great lengths to bother people. These include asking them if they are turtles, like fishsticks, or saying "India". On anonymous video chatting sites, they go to even greater lengths to bother people, making up entire songs and others to bother people. Many examples of trolling can be found on Art of Trolling, a site hosted by the cheezburger network.

Forum troll: Your a faggot so i dont have to aknolege u!111!!!!!
Chat troll: My good sir, it appears that there is a monstrous turtle behind you
Video chat troll: I'm going to dress like a nude girl from behind, but turn around and freak people out when I'm really a guy!

The Urban Dictionary T-Shirt

Ad hominem attack directed at anyone whose opinion is different from your own.

Someone actually responded to my comment on youtube with an opinion that's different from mine. I called him a troll and told him to go f himself. And, just like a troll, he refused to answer my profound political retort.

The Urban Dictionary T-Shirt

A person who posts things on some online community setting that are designed to purposefully assault and offend large numbers of people for no reason other than the Troll's bias against them, hatred for them or personal amusement at annoying them.

Guy 1: Have you been on Encyclopaedia Dramatica?
Guy 2: No!! That place is for Trolls!!
Troll: OMG JOO PEDOPHILE!! U R TEH GAY 1D10T!! G0 DIE JOO FAIL!! LULZ IS GUD ROFLMAO!!
Guy 1: Ah. I see what you mean.